Peltigera spp.
Peltigera lichen typically have large, shield-shaped “leaves” (thalli), which lack a cortex on the underside. The cortex is the part of the lichen that imparts colour, so the underside of Peltigera thalli is noticeably white or paler than the topside.
For a discussion of Australian Peltigera, including a key to the identification of individual species, see this article by Simone H.J.J. Louwhoff in 'Flora of Australia'